Inhibition by yohimbine of the calcium-dependent evoked release of [3H]GABA in rat and mouse brain slices in vitro

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

Alpha-2 adrenoceptor activity affects propofol-induced sleep time

alpha(2) Adrenoceptor activity is involved in the mechanism of anesthesia. Clonidine, a alpha(2) adrenoceptor agonist, and yohimbine, a alpha(2) adrenoceptor antagonist, increase and decrease barbiturate-induced sleep times. In this study, we examined the effects of these drugs on propofol-induced sleep time. One-hundred-eighteen male Wistar rats weighing 320-400 g were used. Rats received saline, yohimbine (1, 0.1, or 0 mg/kg), or clonidine (300, 30, 3, or 0 microg/kg) intraperitoneally followed by 60 mg/kg of propofol in various combinations. In two series of experiments, either sleep time or prefrontal cortex norepinephrine release (microdialysis) was measured. One milligram/kilogram of yohimbine decreased propofol-induced sleep time to approximately 70% of control, and this was accompanied by an increase in perfusate norepinephrine of approximately 240% of control. Clonidine increased sleep time approximately 260% (300 microg/kg) and approximately 170% (30 microg/kg), and this was accompanied by a decrease (approximately 60% in both doses) in perfusate norepinephrine. In the present study, we show that the alpha(2) antagonist, yohimbine, decreased and the alpha(2) agonist, clonidine, increased propofol-induced sleep times. These changes were essentially mirrored in both groups by changes in norepinephrine release in the prefrontal cortex.

IMPLICATIONS

Central alpha(2) adrenoceptor is thought to be involved in several IV anesthetics-induced sleep. In this study, activation of the receptor increased the propofol-induced sleep time, whereas its inhibition decreased the sleep time. The results provide further evidence that the alpha(2) receptor is a good tool to elucidate the mechanism of anesthetics-induced sleep.

The alpha-2 antagonists idazoxan and rauwolscine but not yohimbine or piperoxan are anxiolytic in the Vogel lick-shock conflict paradigm following intravenous administration

The alpha 2 agonist clonidine has been shown to be anxiolytic in a number of preclinical anxiety models. Interestingly, intravenous infusion of the alpha 2 antagonists idazoxan at 10 mg/kg and rauwolscine at 2.24 mg/kg significantly disinhibited lick-shock conflict responding in rats similar to the alpha 2 agonist clonidine (0.022 mg/kg) and the benzodiazepine diazepam (0.5 mg/kg). However, the alpha 2 antagonists yohimbine and piperoxan, the alpha 2 agonists medetomidine, guanfacine, and guanabenz, the non-specific alpha antagonist phentolamine, and the alpha 1 antagonist prazosin did not disinhibit conflict responding in the Vogel lick-shock paradigm. In fact, yohimbine has been shown to be anxiogenic in both animals and man. This may be due to yohimbine's lack of specificity and its ability to inhibit GABAergic release. In addition, all of these agents, except idazoxan, did not increase water consumption in water deprived rats. Idazoxan (10 mg/kg) significantly decreased water consumption by 45%. Therefore, idazoxan increased conflict responding for water reward at a dose (10 mg/kg) which also decreased water consumption in a non-conflict paradigm. These data suggest that agents with selective antagonism at the alpha 2 receptor site may be anxiolytic while agents with less specificity at this site such as yohimbine, piperoxan, and phentolamine are not anxiolytic.

Release of GABA and taurine from brain slices

In brain slices the mechanisms of release of GABA have been extensively studied, but those of taurine markedly less. The knowledge acquired from studies on GABA is, nevertheless, still fragmentary, not to speak of that obtained from the few studies on taurine, and firm conclusions are difficult, even impossible, to draw. This is mainly due to methodological matters, such as the diversity and pitfalls of the techniques applied. Brain slices are relatively easy to prepare and they represent a preparation that may most closely reflect relations prevailing in vivo, since the tissue structure and cellular integrity are largely preserved. In our opinion the most recommendable method at present is to superfuse freely floating agitated slices in continuously oxygenated medium. Taurine is metabolically rather inert in the brain, whereas the metabolism of GABA must be taken into account in all release studies. The use of inhibitors of GABA catabolism is discouraged, however, since a block in GABA metabolism may distort relations between different releasable pools of GABA in tissue. It is not known for sure how well, and homogeneously, incubation of slices with radioactive taurine labels the releasable pools but at least in the case of GABA there may prevail differences in the behavior of labeled and endogenous GABA. It is suggested therefore that the results obtained with radioactive GABA or taurine should be frequently checked and confirmed by analyzing the release of respective endogenous compounds. The spontaneous efflux of both GABA and taurine from brain slices is very slow. The magnitude of stimulation of GABA release by homoexchange is greater than that of taurine under the same experimental conditions. However, the release of both amino acids is generally enhanced by a great number of structural analogs, the most potent being those which are simultaneously the most potent inhibitors of uptake. This may result in part from inhibition of reuptake of amino acid molecules released from slices but the findings may also signify that the efflux of GABA and taurine is at least partially mediated by the membrane carriers operating in an outward direction. It is thus advisable not to interpret that stimulation of release in the presence of uptake inhibitors solely results from the block of reuptake of exocytotically released molecules, since changes in the carrier-mediated transport are also likely to occur upon stimulation. The electrical and K+ stimulation evoke the release of both GABA and taurine. The evoked release of GABA is several-fold greater than that of taurine in slices from the adult brain.(ABSTRACT TRUNCATED AT 400 WORDS)

Opposing effects of dopamine D2 receptor stimulation on the spontaneous and the electrically evoked release of [3H]GABA on rat prefrontal cortex slices

The spontaneous and the electrically evoked release of [3H]GABA were studied in vitro on slices of rat medial prefrontal cortex. The slices were preincubated with [3H]GABA and then superfused with a Krebs' solution. The superfusion with a Ca(2+)-free medium progressively increased the spontaneous [3H]GABA release and strongly decreased the electrically evoked release of [3H]GABA (-65%). The effects of three dopaminergic D2 receptor agonists (RU24926, lisuride and LY171555) were studied on both the spontaneous and the electrically evoked [3H]GABA release. The spontaneous release of [3H]GABA was increased by exposure to each of these three D2 agonists. RU24926 produced a dose-dependent increase from 10(-9) to 3 x 10(-8) M and the maximal effect was totally abolished by the dopaminergic D2 receptor antagonist sulpiride (10(-5) M). With lisuride a progressive increase of [3H]GABA release was observed and a plateau value was reached with concentrations between 10(-7) and 10(-6) M. These effects were totally reversed by 10(-5) M sulpiride. The dose-response relation for LY171555 was bell-shaped, with a maximal effect being obtained with 10(-9) M) LY171555. This effect decreased with a higher concentration (10(-8) M) and finally was no longer observed for 10(-7) M LY171555. The maximal increase induced by LY171555 was totally abolished by 10(-5) M sulpiride. In contrast, the electrically evoked release of [3H]GABA was inhibited by these three D2 agonists. The IC50 value of the inhibition was 4.1 x 10(-8) M for RU24926 and 2 x 10(-7) M for lisuride. Sulpiride (10(-5) M) totally abolished the effect of 10(-7) M RU24926. In the concentration range of lisuride examined, a 50% reduction of the lisuride inhibition was obtained in the presence of sulpiride (10(-5) M). The dose-response curve obtained with LY171555 had a U-shape, with a maximal inhibition reached with 10(-8) M, whereas no effect was observed with 10(-6) M. The inhibition induced by 10(-8) M LY171555 was completely antagonized by 10(-5) M sulpiride. The D2 agonist-induced inhibition of the electrically evoked release of [3H]GABA was mimicked by dopamine endogenously released by 10(-5) M amphetamine. This effect was reversed by 10(-5) M sulpiride. Our data provide further evidence for a dopaminergic control of GABA interneurons in the prefrontal cortex. This regulation implies the activation of D2 dopaminergic receptors. The possible mechanisms underlying the opposite effects of D2 agonists on the spontaneous and the electrically evoked release of [3H]GABA are discussed.

Calcium-independent GABA release from striatal slices: the role of calcium channels

We have investigated the role of Ca2+ and Ca2+ channels in the modulation of GABA release. Brain slices prepared from rat striatum were preincubated with [3H]GABA, superfused with Krebs bicarbonate buffer, and exposed to electrical field stimulation (2 Hz for 3 min). Tritium efflux was measured as an index of GABA release. Both resting and evoked efflux were greatly accelerated by deleting Ca2+ from the medium and adding EGTA (1 mM). However, when the concentration of Mg2+ in the buffer was elevated to 10 mM, no effect of the Ca2(+)-deficiency was observed on resting release and its impact on evoked overflow was diminished. Moreover, addition of verapamil (10 microM), a Ca2+ channel blocking agent, reduced evoked overflow even in the absence of external Ca2+, while 4-aminopyridine (10 microM), a K+ channel inhibitor, enhanced GABA efflux in normal buffer but had no effect in the absence of Ca2+. Finally, we have shown previously that nipecotic acid, an inhibitor of high affinity GABA transport, increases GABA overflow in normal buffer, but blocks it in Ca2(+)-free buffer. Collectively, these results suggest that Ca2+ channels may play two roles in the regulation of depolarization-induced GABA release. Firstly, these channels permit a depolarization-induced influx of Ca2+ which then promotes GABA release. In addition, these channels influence GABA release through a mechanism that does not involve external Ca2+. Although the precise nature of this latter involvement is unclear, we propose that the Ca2+ channels serve to permit an influx of Na+, which in turn promotes Ca2(+)-independent release through an influence on the high affinity GABA transport system.

Effects of 5-HT1A receptor agonists and NMDA receptor antagonists in the social interaction test and the elevated plus maze

The effects of several 5-HT1A agonists and excitatory amino acid antagonists were compared to the standard benzodiazepines, diazepam and chlordiazepoxide (CDP) in two assays predictive of anxiolytic activity, the social interaction and elevated plus maze procedures. Indicative of anxiolytic effects the 5-HT1A agonists, buspirone, gepirone and 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) all significantly increased social interaction time and open arm exploration time in the social interaction and elevated plus maze procedures, respectively. Likewise, anxiolytic activity in these assays were also produced by the competitive N-methyl-D-aspartate (NMDA) antagonists, 2-amino-5-phosphonovaleric acid (AP-5), 2-amino-7-phosphonoheptanoic acid (AP-7), 3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) and the non-competitive NMDA antagonist, (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801) while NMDA produced anxiogenic effects. Furthermore, the anxiolytic effects of these agents were of equal magnitude to the benzodiazepines. These two classes of compounds were differentiated in the yohimbine-induced seizure assay, with the NMDA antagonists dose dependently antagonizing seizures similar to the benzodiazepines while the 5-HT1A agonists were inactive. These results suggest that the 5-HT1A agonists and the NMDA antagonists may be potential non-classical anxiolytic agents with different mechanisms of action.

Characterization of [3H]GABA release from striatal slices: evidence for a calcium-independent process via the GABA uptake system

GABA can be released by depolarization even in the absence of external Ca2+. To investigate the underlying mechanism of this phenomenon, GABA release was studied using slices prepared from rat striatum. Slices were preincubated with [3H]GABA in the presence of beta-alanine and superfused with Krebs buffer. Total tritium efflux was measured as an index of GABA release. Electrical stimulation at 2 Hz for 3 min elevated resting tritium efflux approximately two-fold. Decreasing external Ca2+ to 0.1 mM increased basal tritium efflux and reduced electrically evoked overflow, while omitting Ca2+ entirely (and adding 1 mM EGTA) increased both basal efflux and evoked overflow. Tetrodotoxin (5 microM) abolished the evoked release of tritium but did not affect the resting outflow in either normal or Ca2+-deficient conditions. In the presence of normal Ca2+, nipecotic acid (0.1-1 mM), an inhibitor of GABA transport into neurons as well as glia, enhanced both spontaneous efflux and evoked overflow of tritium. Nipecotic acid also increased spontaneous release when external Ca2+ was reduced or removed; however, under these conditions electrically evoked overflow was reduced. These results suggest that the electrically evoked release of [3H]GABA from striatal slices is of neuronal origin, but can occur in part in the absence of external Ca2+. They further suggest that this Ca2+-independent release, which may co-exist with the Ca2+-dependent release, takes place via the same carrier system utilized for high-affinity GABA uptake.

Paradoxical antagonism by bicuculline of the inhibition by baclofen of the electrically evoked release of [3H]GABA from rat cerebral cortex slices

The presynaptic regulation of the electrically evoked release of [3H]GABA was studied in the rat cerebral cortex. Among the GABA receptor agonists tested (GABA, SL 75102, muscimol, THIP, isoguvacine, (+/-)-baclofen), only (+/-)-baclofen inhibited the stimulation-evoked release of [3H]GABA. This effect of baclofen was stereoselective in favor of the (-) enantiomer. The inhibition by (+/-)-baclofen of the electrically evoked release of [3H]GABA was antagonized by bicuculline and picrotoxin. Our results suggest that the release of [3H]GABA in vitro can be modulated by a receptor-mediated mechanism which is sensitive to baclofen, bicuculline and picrotoxin but not to GABA, muscimol or THIP.

Modulation of gamma-[3H]aminobutyric acid release from rat cortical slices by alpha 2-adrenoceptors

Modulation of gamma-aminobutyric acid (GABA) release by alpha 2-adrenoceptor agonists has not been consistently demonstrated. This could be due to high levels of norepinephrine (NE) concomitantly evoked by stimulation parameters needed for GABA release. In the present experiments, NE release was preferentially decreased by omission of calcium (Ca2+) and alpha 2-modulation of [3H]GABA release from cortical slices was measured. The antagonist rauwolscine increased only Ca2+-dependent [3H]GABA release, while the agonists guanabenz and clonidine inhibited only Ca2+-independent GABA release. These results suggest that release of endogenous NE diminished the effect of alpha 2-agonists but reveals the effect of antagonists and support the hypothesis that endogenous NE inhibits GABA release in cortex.

A search for receptors modulating the release of gamma-[3H]aminobutyric acid in rabbit caudate nucleus slices

Various putative striatal transmitters and related compounds were studied for their effects on the release of gamma-aminobutyric acid (GABA) from slices of the head of the rabbit caudate nucleus. The slices were preincubated with [3H]GABA and then superfused and stimulated electrically at 5 or 20 Hz. Aminooxyacetic acid was present throughout. The main changes observed were the following. The basal and, less consistently, the electrically evoked overflow of [3H]GABA were enhanced by 3,4-dihydroxyphenylethylamine (dopamine), an effect not blocked by cis-flupentixol or domperidone and not mimicked by apomorphine and D1-selective agonists. The electrically evoked overflow was diminished by 5-hydroxytryptamine (serotonin); the inhibition was prevented by methiothepin. The basal but not the electrically evoked overflow was enhanced by carbachol; acetylcholine and nicotine also accelerated the basal outflow whereas oxotremorine caused no consistent change; the effect of carbachol and acetylcholine were blocked by hexamethonium but not by atropine or by tetrodotoxin. These findings indicate that the GABA neurons in the caudate nucleus may be stimulated by dopamine, although the receptor type involved remains unclear; inhibited by serotonin; and stimulated by acetylcholine acting via a nicotine receptor. However, all drug effects observed were relatively small. No evidence was obtained for autoreceptors, alpha 2-adrenoceptors or receptors for opioids, adenosine or substance P at the GABA neurons.